Inconsistency between parameter names for flags and include directories

[GPMueller]

Currently, the config keywords are

• include_directories
• public_include_directories and
• flags
• interface-flags
• public-flags

Note also that the documentation currently is quite unclear: https://clang-build.readthedocs.io/en/latest/user_guide/inheritance.html shows public-include-directories as title, but public_include_directories in the code example, while public-flags is the correct title but no code example is given. On top of this, in our internal file-dicts the public include directories key is called 'include_directories_public'...

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To make everything consistent, we should rename

• interface-flags -> interface_flags
• public-flags -> public_flags
• file-dict key 'include_directories_public' -> 'public_include_directories'

And to improve the documentation, we should

• add a new section (next to User's Guide?) called "Config file parameters", containing a reference of how/what one can set in a clang-build.toml
• correct the inheritance documentation titles
• extend the inheritance documentation by giving code examples